I want to connect multiple (up to 4) MAX30100 or MAX30102 oximeter sensors to an Arduino via I2c. But I could not find a solution to change/assign the I2C address.
LTC4316 is what you need. I haven't used it yet, but I know it from a project that did: mocap. It is an I2C translator IC.
Of course, if you need those ICs to be far away from each other, use a uC (or a separate Arduino) with all of those, and connect them with a different protocol!
Nyos's suggestion of an I2C address translator is the ticket. I hadn't heard of these chips before, so I started searching. I found that the one they recommended may be being phased out. It also looks like it's surface-mounted. I found a TI data sheet that lists a DIP package chip TCA9548APW that lets you map up to 8 I2C devices to different addresses. You might want to check those out as an alternative.